1. Field of the Invention
The present invention relates to regulated DC power supplies, especially power supplies used for ion lasers and the like.
2. Description of Related Art
The series resonant power supply of the prior art is used for converting an unregulated input, such as may be generated by rectifying a line voltage from an electrical power utility company, to a regulated DC output.
An early reference disclosing the prior art series resonant power supply is Chambers, "A NEW HIGH FREQUENCY RESONANT TECHNIQUE FOR DYNAMIC CORRECTION OF OFF-LINE CONVERTER INPUT CURRENT WAVEFORMS", Proceedings of POWERCON 10, San Diego, Calif. Mar. 22-24, 1987, F-1, pp 1-7.
The basic half-bridge series resonant supply is shown in FIG. 1. The supply receives an unregulated input voltage V.sub.IN across a positive terminal 10 and a negative terminal 11. A first switching element, which in FIG. 1 comprises the silicon controlled rectifier SCR1, is coupled from terminal 10 to a common terminal 12, for switching current flow from terminal 10 to the terminal 12. A feedback diode D1 is coupled from terminal 12 to terminal 10. Likewise, a second switching element, consisting of the silicon controlled rectifier SCR2, is coupled from the terminal 12 to the negative input terminal 11. Feedback diode D2 is coupled from terminal 11 to the common terminal 12. A series resonant circuit from terminal 12 to terminal 13 consists of an inductor L.sub.R, a capacitor C.sub.R, and a primary coil of transformer T1. Terminal 13 is coupled through capacitor 14 to terminal 10 and through capacitor 15 to terminal 11. The secondary coil of the transformer T1 is coupled to a full-bridge rectifying circuit 16. The output of the full-bridge rectifying circuit is coupled across capacitor 17 and supplies the regulated output voltage.
In operation, SCR1 is turned on while SCR2 is off, establishing current flow from right to left through the transformer T1. Then, SCR2 is turned on and SCR1 turned off, establishing current flow from left to right. The resulting alternating current flow in the secondary coil of transformer T1 is rectified and results in a DC output V.sub.OUT.
In order to increase or decrease the output voltage V.sub.OUT, the frequency of switching SCR1 and SCR2 is increased or decreased, respectively. At higher switching frequencies, a higher average current is coupled into the secondary coil of the transformer T1 and higher output voltage results. For lower switching frequencies, less current is coupled into the secondary coil of the transformer T1 and lower output voltage results.
The problem with the prior art series resonant power supplies is that for lower output voltage, the switching frequency of the switching elements falls into the acoustic range, resulting in hum in the power supply. Hum in the power supply can be annoying, particularly in sensitive environments like medical operating rooms using laser surgical techniques.
Accordingly, it is desirable to have a high power regulated power supply with switching frequencies outside the audio range.